This invention relates to apparatus for measuring the yaw rate of a vehicle and an active or semi-active suspension system incorporating such apparatus.
In EP-A-0114757 there is disclosed a vehicle control system for a wheeled land vehicle, comprising wheel suspension devices in the form of double-acting hydraulic actuators by which the wheels are mounted to the body of the vehicle. The suspension devices are controlled by signals representing heave, pitch, roll and warp modes of movement of the vehicle, possibly modified by signals representing the speed and lateral and longitudinal acceleration for the Vehicle, to obtain a desired ride quality and attitude for the vehicle, the signals being derived from appropriate transducers located at appropriate positions on the vehicle.
In U.S. Pat. No. 4,761,022 there is disclosed a similar control system which also includes steering angle and yaw rate sensors on the vehicle, the signals from these sensors being used to control the steering characteristics of the vehicle. As disclosed, the yaw rate sensor can be a rate gyrometer, a vibration gyrometer or an optical fiber gyrometer.
The accuracy to which the vehicle steering characteristics can be controlled depends directly upon the accuracy of the sensors employed, in particular upon the stability of the yaw rate sensor output signal. This feature has, in the past, caused high quality, and therefore high cost, rate sensors to be employed for the stated purpose. Such sensors are generally fragile when unpowered and can have other undesirable features, such as large physical dimensions and high levels of acoustic emission.
There has recently become available a solid state rate gyrometer which is physically small, has minimal acoustic emissions when operating, has been demonstrated to be of rugged construction, and offers the potential for low unit cost in high volume production. All of these features make the gyrometer eminently suitable for application in a vehicle control system as discussed above.
A wheeled land vehicle generally provides a harsh working environment, including large changes in temperature. The output signal of the solid state rate gyrometer has been found to be affected by changes in ambient temperature to an extent that the performance of a vehicle control system which incorporated the sensor could prove to be unacceptable.